1. Field of the Invention
This invention relates to an image processing apparatus such as a facsimile apparatus having at least two binary-conversion processing methods and the method thereof.
2. Description of the Related Art
FIG. 1 is a block diagram illustrating the construction of a facsimile apparatus according to an example of the prior art. The apparatus includes a solid-state image pick-up device 101 such as a CCD for reading an original and outputting an analog image signal indicative of the original, an A/D converter 102 for converting the analog image signal into a digital signal (in which each pixel is represented by n bits) and outputting the digital signal, a gamma (.gamma.) correcting unit 103 for applying a .gamma. correction to the n-bit digital image signal, namely a multivalued image signal, an error diffusing processor 104 for diffusing, to peripheral pixels, an error which occurs when subjecting the n-bit multivalued image signal to binary-conversion processing and outputting an acceptable binary (two-valued) image signal (in which each pixel is represented by one bit), a coding processor 105 for coding the binary image signal as by a modified Huffman (MH), modified read (MR) or modified-modified read (MMR) facsimile coding method, namely a run-length coding method, an operation panel 106 comprising a display for displaying the status of the apparatus and a key input unit which the operator uses to enter instructions, a memory controller 107 controlling the writing or reading of image data relative to an image memory 108, which image memory 108 is for accumulating the image data, and a communication controller (CCU) 109 for transmitting and receiving data via a communication line 120.
In the arrangement described above, the analog image signal, which is obtained by scanning the original by means of the solid-state image pick-up device 101, is converted into n-bit digital data by the A/D converter 102. Next, the n-bit digital signal, namely the multivalued image signal, is acted upon by the .gamma. correcting unit 103 to convert the quantity of light into density, then the signal is subjected to error diffusing processing by the error diffusing processor 104, whereby the signal is converted into a one-bit binary image signal.
This binary image signal is coded by the coding processor 105 in accordance with the above-mentioned run-length coding method, and the signal is accumulated in the image memory 108 as coded data by the memory controller 107. When the operator uses the operation panel 106 to designate transmission at the end of the accumulating operation, the coded data accumulated in the image memory 108 is sent to the CCU 109 via the memory controller 107, and the CCU 109 sends the coded data to another party's apparatus through the communication line 120 in accordance with facsimile transmission procedure.
However, in the example of the prior art described above, the multivalued image signal is coded following the error diffusing processing, and the coded data is accumulated in the memory 108. As a consequence, the following problems arise:
When coding by the run-length coding method is applied to the binary image signal that has been subjected to error diffusing processing, the data compression ratio declines. This results in memory overflow in the image memory 108 and sometimes makes it impossible to transmit an original.
In order to accumulate the coded data in the image memory 108 reliably, the number of pages accumulated at one time is limited. Moreover, in a case where the number of pages accumulated is the maximum possible, communication time is prolonged and the cost of communication becomes quite high.
Furthermore, depending upon the kind of the binary-conversion processing method described above, there is a case in which the pixels of the output image may either concentrate or scatter. And in an ordinary image printing apparatus, the pixels are enlarged somewhat so that mutually adjacent pixels are caused to overlap before the image is printed out. As a consequence, a difference in density occurs.